Directory Structure OS

A directory or storage device can be used in its entirety for a file system.

It can also be subdivided.

For example,

A disk can be partitioned into quarters, and each quarter can hold a file system. 

Storage devices can also be collected together into RAID sets that provide protection from the failure of a single disk.

Partitions:

Partitions are also known as slices or mini-disks.

Volume:

Any entity containing a file system is generally known as a volume.

The volume may be a subset of a device, a whole device, or multiple devices linked together into a RAID set.

Each volume can be thought of as a virtual disk.

Volumes can also store multiple operating systems, allowing a system to boot and run more than one operating system.